Summary: This project addresses the safety of herpesvirus vaccines as follows: 1) study of the mechanisms by which herpesviruses establish latency and reactivate. Studies of the herpes simplex virus LAT regions are defining the specific viral genetic elements that direct the establishment and maintenance of, and reactivation from, latency. Information from these studies is relevant to development of safe herpesvirus vaccines and vectors. In addition, methods are developed to assess the ability of a vaccine or other intervention to influence the establishment of viral latency. These methods are based on quantitative polymerase chain reaction and on the construction of viral mutants that express reporter genes during latency, enabling the easy detection of latent virus. 2) development of assays to clearly differentiate vaccine strain and wild-type chickenpox strains. An assay was developed by sequencing of the vaccine strain genome, and identification of specific differences. This assay correctly identifies VZV strains as vaccine or non-vaccine in 100% of test cases. Availability of improved assays will enable more reliable assessment of potential human to human transmission of vaccine strain, and of vaccine strain reactivation in humans. Identification of these differences may also yield important information regarding the basis for attenuation of the vaccine strain, providing a basis for the performance of additional important studies. As a component of these studies, we also have been examining rapid methods to evaluate the antiviral susceptibility or resistance of vaccine strain viruses. 3) Study and analysis of data related to the chickenpox vaccine to identify safety concerns. New methods to analyze clinical data related to duration of vaccine response and to the ability of the vaccine strain to reactivate are being developed and used to predict the long term efficacy and safety of the vaccine.